The present invention relates to magnetic particles and a process for producing the magnetic particles, and more in detailed, the present invention relates to magnetic particles for a magnetic toner which have a high fluidity, a low oil absorption, an excellent charging stability and a good initial dispersibility, and a process for producing such magnetic particles. The present invention also relates to iron oxide particles and a process for producing the iron oxide particles, and more in detailed, the present invention relates to iron oxide particles having a low oil absorption and an excellent initial dispersibility, which are suited for use as a coloring pigment for coating materials, inks and resins, and a process for producing such iron oxide particles.
Hitherto, a development method of using as a developer composite particles prepared by mixing and dispersing magnetic particles such as magnetite particles in a resin without using a carrier, so-called one-component magnetic toner, has been widely known and used as one of the electrostatic latent image development methods.
With the enhancement of the image qualities such as image density and gradient and the increase of printing speed of the copying machines in the recent years, more improvements of the properties of the magnetic toner used as a developer are demanded. That is, a magnetic toner is required to have an improved developing-stability so as to allow copying with a uniform image density, a high durability so as to enable clear and uniform copying even after repetition of copying, an excellent environmental stability so as to allow normal copying even under the low-temperature and low-humidity condition or high-temperature and high-humidity condition, as well as excellent fluidity and charging stability.
Regarding the durability of magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 4-162051 states: "Further, since the copying machines are trending toward nigher printing speed, a toner is required to satisfy the high-level requirements for high resolution, high-speed development, high durability, etc. . . ."
As to the developing stability and fluidity of magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 53-94932 describes: "Such high-resistance magnetic toner is poor in fluidity because of high resistance and tends to cause uneven development. That is, the high-resistance toner for PPC can retain the electric charges necessary for transfer, but since there remains a small amount of charges in the toner bottle or on the surfaces of the magnetic rolls even in the steps (other than the transfer step) where no charging is necessary, due to the frictional charging or mechanoelectret in the toner producing process, the agglomeration of particles tends to take place, thereby causing lowering of fluidity." and "Another object of the present invention is to provide a high-resistance toner for PPC having an improved fluidity to obtain high-quality indirect photocopies with no unevenness of development, hence with excellent resolution and gradient."
Concerning the environmental stability, the fluidity and the charging stability of magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 63-139367 states: "In the method using such a dry developer, in order to form a high-quality visible image, the developer is required to have a high fluidity and uniform charging properties, and for this reason, it has been generally practiced to add and mix fine silica particles with toner particles. However, since the silica particles are per se hydrophilic, the developer particles incorporated with the said silica particles tend to agglomerate with the aid of moisture in the air, resulting in reduced fluidity of the developer, and in certain cases, the charging efficiency of the developer may even be lowered due to moisture absorption of silica. . . . . . It is required that the good copying quality be maintained even when the copies are left under a high-humidity condition for a long time, and in this respect, the conventional hydrophobic silica particles were unsatisfactory." and "An object of the present invention is to provide a statically charged developer which is stable against changes of environmental factors such as high or low temperature and humidity, and able to maintain and show the good and excellent properties."
There is a close relation between the properties of the magnetic toner and the properties of the magnetic particles dispersed in the magnetic toner. For improving the developing stability, the durability and the environmental stability of the magnetic toner, it is required that the magnetic particles used for the toner have a good miscibility with the resin and be minimized in oil absorption, as pointed out in Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 which states: "Generally, the magnetic particles for such single-component magnetic toner are required to meet the following property requirements: . . . . . . (VII) To have a good miscibility with resin. The toner particle size is usually not more than 10 .mu.m, and the microscopic mixing degree in the toner is of much significance for the toner properties."
As for the fluidity of magnetic toner, since it greatly depends on the surface condition of the magnetic particles exposed on the magnetic toner surfaces, it is strongly required that the magnetic particles are excellent in fluidity.
The charging stability of magnetic toner is greatly dependent on distribution of the charge on the magnetic particles exposed on the magnetic toner particle surfaces, so that it is keenly required that the distribution of the charge on the magnetic particles be minimized.
Further, since the fluidity and charging stability of magnetic toner are greatly dependent on the surface condition and uniformity of the charging of the magnetic particles as mentioned above, it is necessary to maintain the surface condition and charging stability of the magnetic particles in a favorable state throughout the magnetic toner production process. For this, it is emphatically required that the good initial dispersion be secured in kneading of the magnetic particles with resin.
Various attempts have been made for improving the properties of the magnetic particles contained in resin so as to improve the properties of magnetic toner. For example, the following methods have been proposed:
(1) A water-soluble silicate is added in the course of the reaction for producing the magnetite particles of various shapes by a wet process, in which an oxidation reaction is carried out by passing an oxygen-containing gas through a suspension containing colloidal ferrous hydroxide obtained from the reaction of an aqueous ferrous salt solution and an aqueous alkaline solution (Japanese Patent Publication (KOKOKU) No. 3-9045 and Japanese Patent Application Laid-Open (KOKAI) Nos. 3-131863 and 5-72801).
For example, Japanese Patent Publication (KOKOKU) No. 3-9045 discloses "Spherical magnetite particles having bulk density of 0.40 to 1.00 g/cm.sup.3, containing Si in amount of 0.1 to. 5.0 atom % based on Fe and having an excellent temperature stability" and "In a production of spherical magnetite particles which comprises passing an oxygen-containing gas through a ferrous salt reaction solution containing colloidal ferrous hydroxide obtained by reacting an aqueous ferrous salt solution and an alkali hydroxide of an amount of 0.80 to 0.99 equivalent to Fe.sup.2+ in said ferrous salt solution, the magnetite particles production comprising the first stage in which a water-soluble silicate is added in an amount of 0.1 to 5.0 atom % (calculated as Si) based on Fe, to the said alkali hydroxide or the said aqueous ferrous salt solution containing colloidal ferrous hydroxide, and then an oxygen-containing gas is passed through the resultant solution under heating at 70.degree. to 100.degree. C., thereby producing the spherical magnetite particles from said colloidal ferrous hydroxide, and the second stage in which an alkali hydroxide is added in an amount of not less than 1.00 equivalent to residual Fe.sup.2+ and oxidation under heating is further carried out under the same conditions as in the first stage."
(2) A silicon compound is contained in the inside of the spherical magnetite particles and also exposed at the surfaces thereof (Japanese Patent Application Laid-Open (KOKAI) No. 5-213620).
Specifically, the above Japanese Patent Application Laid-Open (KOKAI) No. 5-213620 proposes "Magnetite particles having a silicon compound incorporated therein and exposed at the particle surfaces" and "A process for producing magnetite particles which comprises adding a silicon compound to a solution containing a ferrous salt as a main ingredient, mixing an alkali in an amount of 1.0 to 1.1 equivalent, carrying out an oxidation reaction while maintaining pH at 7 to 10, supplying deficient iron in an amount of 0.9 to 1.2 equivalent to the initial alkali in the course of reaction, and continuing the oxidation reaction at pH 6 to 10."
(3) An a hydroxide of aluminum and/or an oxide of aluminum are deposited on the surfaces of the magnetite particles of various shapes produced by a wet process (Japanese Patent Application Laid-Open (KOKAI) Nos. 54-139544, 61-53660, 2-73367 and 4-162051).
For example, Japanese Patent Application Laid-Open (KOKAI) No. 61-53660 proposes "A process for producing a magnetic developer which comprises mixing a magnetic powder with a binder resin and granulating the obtained mixture, characterized in that alumina is deposited on the surface of the magnetic powder to an amount of 0.1 to 1%."
(4) Fine zirconia particles are deposited on the magnetic particle surfaces, and an oxide layer containing Al and/or Si is formed on the zirconia particle-surfaces of the magnetic particles (Japanese Patent Application Laid-Open (KOKAI) No. 1-298030).
Specifically, Japanese Patent Application Laid-Open (KOKAI) No. 1-298030 discloses "Magnetic particles in which fine zirconia particles or non-magnetic fine ferric oxide particles are present between the magnetic particle surfaces and an oxide Or hydroxide layer containing Al and/or Si and covering said particle surfaces" and "A process for producing magnetic particles which comprises preparing a mixed suspension containing magnetic particles and fine zirconia particles or non-magnetic fine ferric oxide particles and having a pH of not more than 4 or not less than 9, adding an acid or alkali solution thereto to adjust the pH to 6.5 to 8.5, thereby causing deposition of the said fine zirconia particles or non-magnetic fine ferric oxide particles on the surfaces of said magnetic particles, then adding an alkali solution to the suspension containing said magnetic particles having said fine zirconia particles or non-magnetic fine ferric oxide particles deposited thereon to prepare a suspension with a pH of not less than 10, adding an Al-containing compound and/or a Si-containing compound to said suspension, and further adding thereto an acid to adjust the pH to 6.5 to 8.5, thereby forming an oxide or hydroxide layer containing Al and/or Si on the surfaces of said magnetic particles having said fine zirconia particles or non-magnetic fine ferric oxide particles deposited thereon."
(5) The magnetite particles of various shapes produced by a wet process are compressed by using a wheel-type kneader or an attrition mill (Japanese Patent Application Laid-Open (KOKAI) Nos. 3-131863 and 3-131865).
As means for improving the properties of magnetic toner itself, the following methods are known:
(6) A mixture of hydrophobic magnetite particles and non-magnetic acicular metal oxide or hydrous iron oxide particles is dispersed in a resin (Japanese Patent Application Laid-Open (KOKAI) No. 2-97968).
(7) A mixture of magnetite particles, .alpha.-hematite particles and .gamma.-hematite particles is dispersed in a resin (Japanese Patent Application Laid-Open (KOKAI) No. 63-178256).
The magnetic particles with a high fluidity, a low oil absorption, an excellent charging stability and a good initial dispersibility are most strongly required at present, but such magnetic particles are still unavailable.
The magnetic particles obtained from the above-described methods (1) and (3) are satisfactory in miscibility and dispersibility in resin, but are incapable of improving fluidity and charging stability to a satisfactory degree.
The magnetic particles obtained from the method (2) are indeed improved in fluidity, but their oil absorption is as high as not less than 22 cc/100 g, which indicates poor miscibility and dispersibility with resins.
The method (4) can not improve the initial dispersibility to a satisfactory degree.
The method (5), although effective for improving the miscibility and the dispersibility, is unable to improve a fluidity and a charging stability.
The method (6) is effective for stabilizing the charging amount of the toner when using hydrophobic magnetite, but this method does not improve the fluidity. In this method, the non-magnetic particles are simply mixed in a resin.
The method (7) is incapable of improving the fluidity.
Thus, it is strongly demanded to provide magnetic particles which has a high fluidity, a low oil absorptivity, an excellent charging stability and a good initial dispersibility.
On the other hand, hitherto, the iron oxide particles have been popularly used as a coloring pigment for coating materials, inks and resins by dispersing in vehicle or mixing with a resin, since the hematite (.alpha.-Fe.sub.2 O.sub.3) particles assume a reddish to dark purple color, the maghemite (.gamma.-Fe.sub.2 O.sub.3) particles assume a brown color and the magnetite (FeO.sub.x .multidot.Fe.sub.2 O.sub.3, &lt;.times..ltoreq.1) particles assume a black color.
Recently, from the standpoint of an improvement of working efficiency aimed at energy saving and an improvement of coating properties, the request for better dispersibility of the iron oxide particles in vehicles in production of coating materials has become more and more strong.
In production of coating materials, the dispersibility of the pigment particles in the vehicle becomes a very important factor that governs working efficiency in the production process and decides the coating properties of the product.
This fact is referred to in the Bulletin of Japan Society of Color Materials, Vol. 49 No. 1 (1976), page 8, which states: "It is not too much to say that most of the properties to be possessed by the coating films are decided by dispersibility of the pigment in the coating film. The theories teach that the good dispersibility of the pigment in the coating film makes the color tone vivid and improves the innate basic properties of the pigment such as tinting strength and hiding power. It also improves the luster of the coating film, the vividness, the mechanical properties, the impermeability to moisture, etc., which leads to improved durability of the coating film. It is thus understood that the dispersibility of the pigment in the coating film is an important factor that decides the properties of the coating film."
Observing the course of dispersion of the iron oxide particles in a vehicle or resin, it is noted that the particle dispersion process is diversified, and that the difference in manner of dispersion greatly affects the coating properties and the resin-surface characteristics even if the degree of finally reached dispersion is the same.
Oil absorption means the affinity of the iron oxide particles for linseed oil. The lower the oil absorption, the more improved is the finally reached dispersibility, which leads to improvement of the coating properties such as a tinting strength. However, in case where the initial dispersion is bad even if oil absorption is low, since the particle agglomerates are hard to break up, the iron oxide particles tend to remain as large agglomerates to the last to cause reduction of the tinting strength, etc., especially when a machine with weak dispersing force is used for preparation of the coating material or when the dispersing treatment time is short.
Thus, it is required to provide iron oxide particles having a low oil absorption and an excellent initial dispersibility. Improvement of the initial dispersibillty is also strongly desired from the standpoint of improvement of the working efficiency in answer to the recent requirement for energy saving.
Various techniques for coating the particle surfaces with organic or inorganic compounds have been proposed for improving the dispersibility of the iron oxide particles. For example, there have been disclosed the following methods: (i) A method of mixing ferric hydroxide or hydrous ferric oxide with a ferric oxide pigment (Japanese Patent Application Laid-Open (KOKAI) No. 51-124124); (ii) A method of adhering a higher fatty acid to the surface of a metal hydroxide deposited on the surface of inorganic pigment particles (Japanese Patent Application Laid-Open (KOKAI) No. 52-37930); (iii) A method of subjecting the surfaces of .alpha.-Fe.sub.2 O.sub.3 particles with ferric hydroxide to silane coupling-agent treatment (Japanese Patent Application Laid-Open (KOKAI) No. 55-94968).
As described above, the iron oxide particles with a low oil absorption and an excellent initial dispersibility are the most acutely required in the art, but any of the conventional iron oxide particles such as mentioned above can hardly be deemed as well satisfying the said property requirements.
According to the method (i) mentioned above, in which ferric hydroxide or hydrous ferric oxide is deposited as a layer in an amorphous state on the ferric oxide pigment particle surfaces, the oil absorption and initial dispersion are unsatisfactory as shown in the Comparative Examples.
The method (ii) mentioned above is intended to improve an affinity between the inorganic pigment particles and the higher fatty acid coated on their surfaces by depositing a metal hydroxide as a layer on the said inorganic pigment particle surfaces.
The method (iii) is also intended to improve an affinity between the pigment particles and the silane coupling agent coated on their surfaces by forming a layer of ferric hydroxide on the said pigment particle surfaces.
As seen from the above, it is strongly demanded to provide the iron oxide particles with a low oil absorption and an excellent initial dispersibility.
As a result of studies undertaken by the present inventors to overcome the foregoing problem of the related arts, it has been found that by reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe.sup.2+ in the said ferrous salt solution wherein a water-soluble silicate is previously added in an amount of 0.4 to 4.0 mol % (calculated as Si) based on Fe, to the said aqueous alkali hydroxide solution or the obtained ferrous salt reaction solution containing colloidal ferrous hydroxide, passing an oxygen-containing gas through the ferrous salt-reaction solution containing colloidal ferrous hydroxide at a temperature of 85.degree. to 100.degree. C. to carry out the oxidation reaction, thereby producing Si-containing magnetite particles, then adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to residual Fe.sup.2+, to the suspension after completion of the oxidation reaction, further carrying out an oxidation reaction at 85.degree. to 100.degree. C. to produce the spherical magnetite particles containing silicon element, then adding a water-soluble aluminum salt to the residual silicate-containing alkaline suspension wherein the said spherical magnetite particles containing silicon element have been formed, so that the amount of said aluminum salt becomes 0.01 to 2.0 wt % (calculated as Al) based on the produced particles, then adjusting the pH of the solution to 5 to 9 to forms hydrous coprecipitate composed of silica and alumina on the surfaces of the said spherical magnetite particles containing silicon element, then filtering, washing with water and drying the precipitate, and if necessary subjecting the thus obtained dry particles to compaction, shearing and spatula-stroking by a wheel-type kneader or an attrition mill, the magnetic particles obtained have an excellent fluidity, and are low in oil absorption and also excellent in charging stability; or that by blending 0.1 to 12 parts by weight of non-magnetic fine iron oxide particles and/or non-magnetic fine hydrous iron oxide particles with 100 parts by weight-of the iron oxide particles having an area-average diameter of 0.1 to 1.0 .mu.m, and subjecting the blend to compaction, shearing and spatula-stroking by a wheel-type kneader or an attrition mill, the obtained iron oxide particles are low in oil absorption and have an excellent initial dispersibillty. The present invention has been achieved on the basis of these findings.